1. Field of the Invention
The present invention relates to recording and playback of digital video stream and playback method thereof, and more particularly, to a digital video record/playback apparatus and playback method thereof.
2. Discussion of the Related Art
Generally, data formats of the MPEG (mobbing picture experts group) standard used for compression and transmission of digital video follow either a program stream or a transport stream. The program stream is used in storing digital video having less transmission error in a storing medium, whereas the transport stream using fixed-length packets is used for digital broadcasting having a relatively unstable transport channel and the like. A system, i.e. ISO/IEC 13818-1 system, for transmitting digital video by parsing it into uniform-unit packets is advantageous in error reduction and detection features.
As an apparatus for recording previously and playing back a transmitted transport stream of digital broadcasting, we can consider a device of converting to record a transport stream to a program stream to facilitate playback by extracting appended information required for the playback at a time point of recording digital video and using a player for a conventional storage device. For instance, a digital broadcast stream is converted to a program stream for playback facilitation of a DVD (digital versatile disc) player and is then played back on a display device in accordance with a variety of playback techniques using DVD appended information.
Yet, a general transport stream storage device firstly extracts a number of information required for playback without additional process and device for conversion to a program stream and then inserts timestamps therein to record in a storage device. In doing so, the information required for the playback includes a starting location of picture, a picture type, an encoded frame rate, a bit rate, etc. In playback, such a kind of information is used for a trick play at high or low speed and the like in accordance with an output mode setup.
U.S. Pat. No. 6,16,843 entitled ‘Recording and playback of audio-video transport streams’ proposes an apparatus and method for playing back a transport stream at a normal speed using a timestamp after a series of recording process for inserting the timestamp into the transport stream using PCR (program clock reference).
In the above-explained playback, a digital video stream recorded in the storage device enables to implement the trick play according to various output mode setups, which is proposed and disclosed in U.S. Patent Public Laid-Open Ser. No. US2002/044761 A1 entitled ‘Transmitting apparatus of image information, transmission system, and transmitting mode’.
FIG. 1 is a block diagram of an image information transmitting device disclosed in the U.S. Patent Public Laid-Open Ser. No. US2002/044761 A1. In case of outputting an MPEG stream of trick play, such an output mode as a fast forward playback, fast reverse playback, slow playback, and the like s determined by a control of an output control unit 16. The determined output mode is outputted to an interpret/re-record unit 15. The interpret/re-record unit 15 reads the stream by picture unit, interprets a picture header, and carries out a trick play processing according to output mode information determined in the output control unit 16.
For instance, when a stream displaying the same picture multiple times by slow playback, a skip P picture (hereinafter abbreviated Ps) is used. Namely, if the picture to be displayed is an I or P picture, the skip P picture is outputted to keep being displayed for the original picture. In other words, in case of repeating to display the I picture, ‘IPsPsPs . . . ’ is outputted in place of ‘IIII . . . ’. And, in case of repeating to display the P picture, ‘PPsPsPs . . . ’ is outputted in place of ‘PPPP . . . ’.
In doing so, when the picture is outputted, two kinds of processing are carried out for the conversion to bit streams suitable for the MPEG standard.
First of all, a value of temporal_reference within the picture header is re-recorded as an accurate value. The temporal_reference indicates a display sequence of the picture within GOP (group of picture). Hence, if the temporal_reference value is outputted without being re-recorded, it violates the MPEG standard.
Secondly, a value of vbv_delay (accumulation quantity of a virtual input buffer of decoder) within the picture header is re-recorded as ‘0xFFFF’. This value is a code indicating a null of the vbv_delay. In doing so, the re-recording is carried out because a wrong result is brought about in case of using an original value of the vbv_delay is used as it is, since the picture sequence is varied from an original one on trick play.
Thus, the re-recorded trick play stream is decoded by a general video decoder to be outputted to a screen.
For this, additional devices, i.e., interpret/re-record unit 15 and skip P picture generating unit 18, indicated by dotted lines in FIG. 1 are needed.
The skip P picture generating unit 18 is a unit of generating arbitrarily added P pictures to regulate a frame rate of fast or slow playback in a trick play mode.
In this case, a first macro block of a picture slice is encoded as No MC Not Coded MB (macro block) and the rest macro blocks of the slice are encoded as Skip MB. The No MC (i.e., prediction between simple frames) Not Coded (i.e., having no DCT coefficient) macro block has a motion vector (0, 0) and is a macro block where a motion compensation differential value, i.e., inverse discretion cosine transform (IDCT) coefficient, fails to exist. And, the skip macro block (SB) in the P picture has a motion vector (0, 0) like the No MC Not Coded macro block and is processed as having no motion compensation differential value.
When the skip P picture proposed by the related art is transferred to a video decoder to be decoded accordingly, it brings about an effect that the previous I or P picture is repeatedly played back. In the related art, the skip P picture is encoded into the above-explained picture structure and recorded in a memory device. The skip P picture is then transferred to the video decoder in case of being needed as the trick play mode is set up.
Most of bit data of the skip P picture are encoded into skip macro blocks not to thereby occupy a quite great bit quantity. In spite of small quantity, needed are a memory device for recording the skip P picture and an additional process of reading picture data from the memory device to transfer to the video decoder. Moreover, added are steps of demultiplexing the transport stream to correctly reset the values of the temporal_reference and vbv_delay, decoding the demultiplexed transport stream into PES (packetized elementary stream), and decoding the PES into ES (elementary stream).
Besides, in case that the trick play mode is a slow mode, in the related art shown in FIG. 1, a copy of a B picture is transferred to the video decoder, in which an additional process of reading from the storage device a previous B picture to transfer is needed as well.
FIG. 2 and FIG. 3 illustrate a trick play method according to a related art. FIG. 2 shows a case that the trick play mode is set to a fast forward playback mode, in which I and P pictures are just selected to transfer from an original video stream to cope with a frame rate of 1.5× speed playback and a skip P picture is added in the middle to be transferred to the video decoder. The skip P picture added to a location of which temporal_reference is 1 repeats the I picture at a location of which temporal_reference is 0 to display on the display device.
FIG. 3 shows a case that a trick play mode is set to a slow forward playback mode, in which skip P picture and copied B picture are added to an original video stream to be transferred to the video decoder in order to implement a frame rate of 0.5× speed. And, the copied B picture plays a role in repeating a previous B picture to be displayed on the display device.
However, in the related art shown in FIG. 1, additionally needed to implement the trick play are the memory device for storing the skip P picture as well as the process for reading from the memory and storage devices the skip P picture and copied B picture to transfer to the video decoder.
Moreover, the related art additionally needs a mandatory process of updating the picture header information including temporal_reference and the like.